Re: [RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections

[Peter Zijlstra]

On Mon, Feb 29, 2016 at 09:12:20AM +0800, Boqun Feng wrote:

> The problem is:
> 
> Currently there is no way to know which rcu_dereference() and its
> friends a rcu_read_lock() or one of its friends is protecting. And
> the lack of such information is a pain for kernel developers and
> reviewers to understand the purpose of a certain rcu_read_lock().

If I have to go run a kernel and look at /proc data in order to complete
a review the patch will fail review, end of story.

> > Secondly, I'm not sure a random list of data is going to help anybody.
> > 
> 
> One possible usage might be:
> 
> If someone is going to refactor a piece of code which originally uses
> RCU for synchronization, and now he wants to switch to other
> synchronization mechanism for some data because of some reason(for
> consistency or writer side latency guarantee, etc.). Before he makes the
> change, he needs to know if this rcu_read_lock() also protects other
> data, and this is not an easy job and I don't think we have anything
> helpful for this. With RCU_LOCKED_ACCESS, he can build the kernel with
> RCU_LOCKED_ACCESS enabled, run some workloads and get the information
> from a /proc file. In this way, RCU_LOCKED_ACCESS is helpful to
> developers who want to know the information of the associations between
> rcu_read_lock() and rcu_dereference().

So 'possible', but will anyone actually do this? `

> > The whole point of lockdep is that it tells you if you do broken. This
> > doesn't, and cannot, because it lacks information to verify against.
> > 
> 
> Yes.. RCU_LOCKED_ACCESS is not a validator, so it cannot be as useful as
> lockdep for finding bug.

I would try really _really_ hard to make it a validator, those are so
much more useful.

One could for example allow something like:

	rcu_read_lock();
	rcu_annotate(&var->field);

	foo();

	rcu_read_unlock();

As an alternative to the syntax suggested by Ingo. This would allow
keeping the existing rcu_read_lock() signature so you don't have to
force update the entire kernel at once, while also (easily) allowing
multiple variables. Like:

	rcu_read_lock();
	rcu_annotate(&var->field);
	rcu_annotate(&var2->field2);

You can then have a special rule that if a particular RCU section has an
annotation, any rcu_dereference() not matched will field a warning. If
the annotation section is empty, nothing.


> > So I'm still not sure this is useful. Also, I would argue your code has
> > problems if you cannot even find your rcu_read_lock().
> > 
> 
> I think what you mean here is, for example, the case where we use
> preempt_disable() instead of rcu_read_lock_sched() to pair with
> synchronize_sched(), right?

No, I was more like:

	rcu_read_lock();
	foo()
	  bar()
	    var->func();
	      obj->func();
	        whatever();

and you're looking at a change to whatever() and wonder where the heck
the corresponding rcu_read_lock() lives and if we're having it held at
all.

> If so, sure, this series cannot do anything about this, however, this

Why not? Can't you stick whatever you need into preempt_disable() ?
kernel/sched/core.c:preempt_count_{add,sub}() are there for a (debug)
reason.

> If you think this series deserves a reconsideration I will send out a
> new version with more text about what's the problem and why this is
> useful.

So please make it a validator. That's so much more useful.